Conveyor belts made from PTFE resins are used in many applications around the world. PTFE offers excellent chemical resistance properties, high temperature capabilities, and good release characteristics.
Because many of the applications rely on heat being transferred through the belts, belt thicknesses are preferably kept to a minimum. Thicknesses typically range from as low as 5 mils to possibly as high as 20 mils. The lower thicknesses are particularly critical in food applications, where often the conveyor belts are operating at temperatures as high as 500° F. to fully process the food.
PTFE/fiberglass composites are popular conveyor belts used in food service. Fiberglass, a high modulus and high temperature material, offers very high strength properties at thin dimensions.
The large majority of PTFE/fiberglass conveyor belts in food cooking applications fall in a thickness range of around 10 mils. A typical construction consists of a 6 mil woven fiberglass fabric combined with PTFE resins. The PTFE resins are almost always applied as coatings, but may also be applied as some combination of coatings and laminated films.
While it is well understood that PTFE/fiberglass composites bring many advantages to the conveying of food products, experience has shown that in many industrial applications these advantages are offset by serious drawbacks. For example, the high modulus fiberglass material does not stretch. As a result, most PTFE/fiberglass conveyor belts possess little elongation, which causes belt tracking problems.
Secondly, because fiberglass is brittle, it is vulnerable to mechanical damage. As a result, in belting applications, a high safety factor in fiberglass material is typically used to ensure a successful operation. As a specific example, in a belting application where the required tensile strength for the conveyor belt may range between 5-10 lbs/inch of width (pli), it is not unusual to find a fiberglass reinforcement with a tensile strength measuring in the hundreds of pli. The additional strength is needed for the belt to survive the mechanical stresses encountered in food processing operations over a long term. The mechanical stresses may result from the hard pressing of flour materials, the scraping of food particles, etc.
Also, the limited adhesion of the PTFE to the fiberglass reinforcement is a weak link for the belt life of the composites in food service. Historically, PTFE/fiberglass belting materials have limited adhesion or peel strength ratings for the PTFE-fiberglass interface of less than 5 pli, typically as low as 2 pli.
Finally, fiberglass is vulnerable to chemical attack by many of the materials used in food preparation and cooking. In the production of the food products on the nominal 10 mil PTFE/fiberglass belting material, some form of hot oil or grease typically comes in contact with the PTFE surface as the food is being cooked. Over time, the hot oil is able to penetrate into the interior of the composite due to the inevitable punctures or holes that develop in the PTFE surfaces during the conveying/cooking operation. History has shown that the processing grease or oil is a chemical threat to the fiberglass reinforcement. Once the hot oil reaches the glass fibers, the conveyor belt typically begins to undergo a serious deterioration in performance.
A 10 mil PTFE/fiberglass conveyor belt in food service generally reveals that it has reached a terminal condition when the PTFE coating/film has lifted from the fiberglass surface in the form of a bubble or blister. Mechanical stresses generated between the PTFE and the fiberglass, as the belting material is being conveyed, certainly contribute to surface abnormalities. However, it is believed that another culprit is the weakening adhesion between the woven fiberglass and the PTFE surface, which is being compromised due to chemical attack of the fiberglass by hot grease. As the bond between the PTFE surface and the fiberglass weakens, the PTFE lifts from the fiberglass, which enables the formation of the blister.
The actual mechanism for forming a blister likely entails the hot oil and its fluid components developing a vapor or gas phase during the cooking operation. The conveyor belt, as it makes a cycle, travels from the hot cooking zone to a cooler non-cooking zone. Any fluids residing within the interior of the belting material will elevate in temperature as they travel through the cooking zone and then cool as they leave the cooking zone. The temperature cycling will generate an expansion/contraction in the belting material profile as the hot fluids contained in the belt begin to boil and expand as vapor. Accordingly, the PTFE surface, which at this point is now serving to confine the fluids within the belt interior, will experience stress from the expanding fluids. The end result is delamination of the PTFE surface from the fiberglass reinforcement and blister formation. It is generally at this point that the conveyor belt is taken off the food cooking machine.
Over time, many attempts have been made to overcome the shortcomings of thin PTFE/fiberglass composites used in conveyor belt service. Special PTFE coatings with superior surface durability have been used to improve the life of the conveyor belts. PTFE films have been laminated to the fiberglass carcass in an effort to provide better resistance to the penetration of the grease and oil chemicals used during cooking.
Modest improvements have been achieved in conveyor belt life on occasions. However, the fundamental tracking and adhesions problems have continued to prevail. The chemical attack of the fiberglass reinforcement also remains as a major problem, in part because of the relatively large amounts of fiberglass that are required for manufacturing the thin, lightweight, belting, materials.
Of late, as disclosed in U.S. Pat. No. 7,673,742, the disclosure of which is herein incorporated by reference, an all-PTFE material has been developed to serve as a conveyor belt for processing food. While the all-PTFE material has provided very significant benefits over certain PTFE/fiberglass belting materials, it has come up short in a number of food processing applications.
For example, the all-PTFE material does not possess the high strength properties of woven fiberglass. Also, conveyor belts made solely from PTFE resins are more likely to have problems maintaining flat, uniform, conveying, surfaces in food production. The stretching of the PTFE material will typically produce some irregular surface features under load and temperature. While many food cooking plants, such as chicken plants, can easily accommodate these surface irregularities, which often include wrinkles, for example, other food processing plants cannot accept the resulting impressions caused by the wrinkles or surface distortions. Thus, for those facilities relying on a very stable, relatively flat PTFE conveyor belt, the all-PTFE conveyor belt is not a candidate.
When foods are cooked by being conveyed through ovens or other like high temperature environments, the process dictates that heat must be transferred through PTFE/fiberglass composite. Thus, the composite must be relatively thin, with most thicknesses ranging between 5-20 mils, and preferably between 10-15 mils. The composite must lay somewhat flat and be free of severe wrinkles and other major defects. It must be strong enough to convey a load, which can vary dramatically from service to service, ranging from a typical 3 to 5 pli to as high as possibly 10 pli. It must be able to withstand exposure to severe pressing and puncturing forces. These and other factors tend to limit the number of raw materials that can be considered for the PTFE/fiberglass conveying belts used in food cooking applications.
The most popular woven fiberglass reinforcement used in the typical food applications is a style commonly referred to in the industry as “7628”. The 7628 style weighs about 6 oz/sq yd and has a thickness of 6 mils. The PTFE surface in a PTFE/fiberglass composite delaminates when the adhesion or peel strength between the PTFE and the woven fiberglass becomes very low. For the style 7628 fabric, the maximum adhesion typically generated between the PTFE surface and the fiberglass substrate typically ranges around 2 pli. Style 128, another 6 oz/sq yd product, is a somewhat similar style of woven fiberglass that is capable of supporting slightly higher adhesion levels. However, because style 128 is substantially more expensive than style 7628, it is less frequently used, even though it offers the possibility of a higher adhesion.
What can be definitely concluded is that the adhesion levels of the PTFE/fiberglass composites currently being employed in food processing applications are not sufficiently high for the extended belt life that most food processors would like to achieve. Those woven fiberglass styles that have the potential of providing for higher PTFE/glass adhesions are typically too thick and heavy to serve in food processing applications. Woven fiberglass styles that are thinner and lighter in weight than styles 7628 and 128 often result in low adhesion levels or are not generally considered adequate due to poor mechanical properties.
Finally, the two main styles, 7628 and 128, consist of tightly woven fiberglass fabrics. In addition to allowing for limited bonding strength to PTFE resins, these fabrics offer no elongation capabilities of any consequence. So, when put into service in belting applications, both products can be very difficult to track on food processing machines.
It will be seen, therefore, that there remains a need for an improved PTFE/fiberglass belting material that: is relatively thin in order to accommodate efficient heat transfer; has the ability to remain adequately flat and stable; is capable of conveying the loads encountered in food service applications; has high adhesion of its PTFE surfaces to the fiberglass fabric; and, has critical elongation properties.
The objective of the present invention is to provide a novel PTFE/fiberglass belting material that meets all of these requirements.